Many of conventional plastic products, especially plastic packaging materials, are discarded soon after use. How to dispose of them is therefore becoming a concern today. Typical packaging plastics include polyethylenes, polypropylenes and polyethylene terephthalate (PET). These plastics tend to release large amounts of heat when burned and thus could damage incinerators when burned therein. Polyvinyl chloride, which is still being widely used, cannot be burned due to its self-extinguishing properties. Many of plastic products, including plastics that cannot be burned, are buried today. But due to their chemical and biological stability, they scarcely decompose in the soil, thus shortening the life of burial sites. It is therefore acutely desired to develop materials that are safe, release less heat when burned and can easily decompose in the soil. Studies for this purpose are now being vigorously made by many researchers.
Polylactic acid is one of such materials. That is, the combustion heat of polylactic acid is less than half that of polyethylenes. It is naturally hydrolyzed in the earth or in the water, and is then decomposed by microorganisms into harmless substances. Studies are now being made to form various products such as film sheets and bottles from polylactic acid.
But packaging films simply formed from polylactic acid are usually too low in gas barrier properties to be used e.g. to store foods. In JP patent publication No. 2003-62933A (“JP'933”); JP patent publication No. 2003-64303A (“JP'303”), JP patent publication No. 11-42752A (“JP'752”) and JP patent publication No. 2003-145677A (“JP'677”), it is proposed to provide a thin inorganic film on a polylactic acid film by e.g. vapor deposition to impart gas barrier properties to the polylactic acid film.
Any of the above references propose to provide a thin inorganic film on a polylactic acid film by vapor deposition to improve gas barrier properties of the polylactic acid film. Specifically, in JP'933 and JP'303, a thin inorganic deposited film is adhered to a polylactic acid film through an anchor layer. The anchoring agent forming the anchor layer is an aliphatic polyester containing an L-lactic acid residue and a D-lactic acid residue in the ratio of 1-9.
In JP'752, an oxide film is deposited on a resin layer containing as its major component a hydroxycarboxylic acid containing lactic acid in its repeating unit. In JP'677, an anchor layer such as a polyester resin is formed on a biodegradable resin film of the polylactic acid family or the polyester family, and then a metallic layer is deposited thereon.